Methods to make thick film single elements and arrays

ABSTRACT

A material for a thick film element is deposited onto a surface of a first substrate to form a thick film element structure having a thickness of between greater than 10 μm to 100 μm. The at least one thick film element structure is bonded to a second substrate. Thereafter, the first substrate is removed from the at least one thick film element structure using a liftoff process which includes emitting, from a radiation source (such as a laser or other appropriate device), a beam through the first substrate to an attachment interface formed between the first substrate and the at least one thick film element structure at the surface of the first substrate. The first substrate is substantially transparent at the wavelength of the beam, and the beam generates sufficient energy at the interface to break the attachment.

BACKGROUND OF THE INVENTION

The present application is directed to piezoelectric material productionand more particularly to a process for manufacturing piezoelectric thickfilm elements and arrays of elements, and structures incorporating suchelements.

Piezoelectric ceramic films, e.g., lead zirconate-lead titanate (PZT)and its modified forms are generally defined as being either thin-filmelements, up to approximately 10 μm in thickness, or thick-filmelements, being approximately greater than 10 μm in thickness. Thin-filmpiezoelectric elements and thick-film piezoelectric elements greaterthan approximately 10 μm thick, can be used in a wide variety ofapplications, including but not limited to microelectromechanicalsystems (MEMS), microfluid pumps or ejectors, such as jet printheads oracoustic ejectors, and ultrasonic transducers.

Unfortunately, elements in the range of greater than 10 μm to 100 μm arenot now able to be produced in high volume with economical yields whichpermit commercialization. Rather, current methods to make the films insuch thickness range are either by polishing the bulk ceramic piecesfrom more than 100 μm down to the required thickness or using a sol-gelhybrid (or composite) process. The first method is a time-consuming andexpensive process which does not lend itself to the making of patternsor arrays. The thick films obtained by the second method have very lowquality, are difficult to be patterned, and the required annealing stepat 500 to 700° C. limits the substrates which may be used. Thus, thereare no cost-effective methods to make high-quality, thick film (greaterthan 10 to 100 μm) individual elements and arrays, with the elementshaving arbitrary shapes and on anykind of substrate including silicon,metal and plastics or epoxies.

For many of these applications, the so called thick films, with thethickness range from greater than 10 to 100 μm, are consideredbeneficial in order to generate a large displacement, apply a largeforce, to provide a suitable working frequency ranges, and to optimizethe performance of actuation or sensing systems. For example, in anexisting piezoelectric inkjet printhead, with a stainless steeldiaphragm having a thickness of 25 to 40 μm, the thickness of thepiezoelectric elements should be about 40 to 70 μm for an optimizeddesign.

Piezoelectric films with the thickness range of greater than 10 to 100μm are also useful for high frequency (20 to 200 MHz) transducers andcatheters used in imaging, such as imaging of arterial walls, structuresin the anterior chamber of the eye, and intravascular ultrasoundimaging.

These applications may find use for both single element transducers andtransducer arrays. For these applications it may be useful to providethe piezoelectric films on polymers, such as some epoxies, which worksas backside materials to absorb or diminish backside ultrasonic wavesfor better image quality, or other advantages.

However, to fabricate piezoelectric films in a greater than 10 to 100 μmthickness range on suitable substrates for such uses is very difficultfor current thin and thick film processes. This is because, thetraditional thin film processes, such as sol-gel processing, sputteringand chemical vapor deposition, can only practically generate films withthickness up to 10 μm range. It is also not efficient to use these thinfilm processes to produce thick films even if they could do so. On theother hand, the traditional thick film processes, such as screenprinting, can produce thick films only on the substrates which canwithstand higher than 1100° C. temperatures because the screen printedfilms have to be sintered at about 1100 to 1350° C. for densificationand to get good properties.

While a sol-gel hybrid (or composite) method, in which ceramic powdersare suspended in a sol-gel solution for spin coating, has been developedat Queen's University of Canada to prepare 0-3 ceramic (powders)/ceramic(sol-gel matrix) composite films with the thickness of 10 to 80 μm onsilicon and metal substrates, there are still several drawbacks for thismethod. First, the film density, and hence the film quality is very lowbecause of low densification process and no grain growth of powdersduring sintering. Secondly, the film is very difficult to etch orpattern due to its inhomogeneous nature in micrometer scale. Thirdly, asthe films have to be sintered at 600 to 700° C., this method can not beused to deposit films on polymers or other substrates which can notwithstand 600° C. or higher.

U.S. Pat. No. 6,071,795 to Cheung et al. provides a method of separatinga thin film of gallium nitride (GaN) epitaxially grown on a sapphiresubstrate. The thin film is bonded to an acceptor substrate, and thesapphire substrate is irradiated by a radiation source (such as a laseror other appropriate device) with abeam at a wavelength at whichsapphire is transparent but the GaN is strongly absorbing, e.g., 248 nm.After the irradiation, the sample is heated above the melting point ofgallium (Ga), i.e., above 30° C., and the acceptor substrate and theattached GaN thin film are removed from the sapphire growth substrate.It was noted that at about 400 mJ/cm², one pulse of the laser wassufficient to separate the epitaxially grown film of GaN from thesapphire substrate. It is also noted in a specific embodiment, the thinfilm of the GaN is grown to a thickness of 3 μm.

It is considered that the high energy levels required for the separationprocess of the thin film GaN, is in part due to the fact that the GaN isepitaxially grown on the substrate, resulting in a degree of latticematching between the GaN film and the sapphire substrate. Thisrelationship results in a strong adhesive energy between the substrateand GaN.

It is therefore deemed desirable to develop a process which caneffectively deposit greater than 10 to 100 μm-thick piezoelectric filmson various substrates (silicon, metals, polymers), where the films canbe easily patterned during the process, and can produce identical,large-quantity, high-quality thick film elements detachable from thesubstrate.

SUMMARY OF THE INVENTION

A method of producing at least one piezoelectric element includesdepositing a piezoelectric ceramic material onto a surface of a firstsubstrate to form at least one piezoelectric element structure. Then anelectrode is deposited on a surface of the at least one piezoelectricelement structure. Next, the at least one piezoelectric elementstructure is bonded to a second substrate, the second substrate beingconductive or having a conductive layer. The first substrate is thenremoved from the at least one piezoelectric element structure and asecond side electrode is deposited on a second surface of the at leastone piezoelectric element structure. A poling operation is performed toprovide the at least one piezoelectric element structure withpiezoelectric characteristics.

In another embodiment, a material for a thick film element is depositedonto a surface of a first substrate to form a thick film elementstructure having a thickness of between greater than 10 μm to 100 μm.The at least one thick film element structure is bonded to a secondsubstrate. Thereafter, the first substrate is removed from the at leastone thick film element structure using a liftoff process which includesemitting,. from a radiation source (such as a laser or other appropriatedevice), a radiation beam through the first substrate to an attachmentinterface formed between the first substrate and the at least one thickfilm element structure at the surface of the first substrate. The firstsubstrate is substantially transparent at the wavelength of the beam,and the beam generates sufficient energy at the interface to break theattachment.

In still another embodiment, a piezoelectric element includes apiezoelectric element structure having a thickness of between 5 μm to100 μm formed by a deposition process. The piezoelectric elementincludes a first electrode deposited on a first surface of thepiezoelectric element structure, and a second electrode deposited on asecond surface of the piezoelectric element structure.

In still a further embodiment of the present application, a device isprovided including a piezoelectric element having a piezoelectricelement structure with a thickness of between 5 μm to 100 μm formed by adeposition process.

SUMMARY OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating preferred embodiments and are notto be construed as limiting the invention.

FIG. 1 is a high level process flow for piezoelectric element productionand direct bonding to a final target substrate or system;

FIG. 2 is a high level process flow for piezoelectric element productionincluding attachment of the piezoelectric elements to a transfersubstrate prior to transfer to a final target substrate or system;

FIG. 3 illustrates a piezoelectric element array on a top surface of acarrier substrate;

FIGS. 4A and 4B show alternative embodiments of a piezoelectric elementarray deposited with electrodes and other thin film metals for bonding,the piezoelectric element array is on a top surface of a carriersubstrate;

FIG. 5A illustrates an embodiment of a bonding of piezoelectric films toa final target which is conductive using a thin, nonconductive epoxybonding containing sub-μm (micrometer) conductive balls;

FIG. 5B shows a nonconductive epoxy bonding process;

FIG. 5C is an enlarged view of a section of FIG. 5B;

FIG. 5D illustrates a bonding of piezoelectric films to a final targetusing thin film intermetallic transient liquid phase bonding;

FIG. 5E depicts an embodiment using separate substrates for depositingof the elements;

FIG. 6A depicts a bonding to a transfer substrate which is conductiveusing a removable conductive tape bonding;

FIG. 6B illustrates a bonding of the piezoelectric films to the transfersubstrate which is an Indium-Tin-Oxide (ITO)-coated glass using thin,nonconductive epoxy bonding containing sub-μm conductive balls;

FIG. 7A illustrates radiation of a beam through the carrier substrateduring a liftoff process;

FIG. 7B depicts a heat transfer for the liftoff process;

FIGS. 8A and 8B are alternative designs of bonding the thick film arrayto a final target substrate or system or to a transfer substrate, withpoling operation;

FIG. 9A illustrates bonding the thick film elements array to a finaltarget system using thin, nonconductive epoxy bonding containing sub-μmconductive balls, where the thick film elements array is bonded to thetransfer substrate using removable conductive epoxy bonding;

FIG. 9B is a bonding of the thick film elements array to the finaltarget system using thin film intermetallic transient liquid phasebonding, where the thick film elements array is bonded to the transfersubstrate using removable conductive epoxy bonding;

FIG. 9C is a bonding of the thick film elements array to the finaltarget system using thin, nonconductive epoxy bonding containing sub-μmconductive balls, where the thick film elements array is bonded to anITO-coated glass using the thin, nonconductive epoxy bonding containingsub-μm conductive balls;

FIG. 9D depicts bonding the thick film elements array to the finaltarget system using thin film intermetallic transient liquid phasebonding, where the thick film elements array is bonded to the ITO-coatedglass using the thin, nonconductive epoxy bonding containing sub-μmconductive balls;

FIGS. 10A and 10B depict alternative embodiments of a final constructedsystem;

FIG. 11 is a chart depicting transmission wavelength of a beam used in aprocess of the present application;

FIG. 12 illustrates a case where removable conductive tape is used inthe configuration of a piezoelectric element array;

FIG. 13 depicts a case of a thin, nonconductive epoxy bonding containingsub-μm conductive balls being used to bond the thick film elements to anITO-coated glass as a transfer substrate;

FIG. 14 illustrates a case of a thick film element being bonded to arigid carrier using a removable tape;

FIG. 15 depicts a structural application of a microfluid pump such as aprinthead in which the piezoelectric elements of the present applicationmay be implemented;

FIG. 16 depicts a hand-held sonar transducer array in whichpiezoelectric elements of the present application are implemented;

FIG. 17 depicts a configuration of an annular transducer array using thepiezoelectric elements of the present application;

FIG. 18 sets forth a mechanical rotating single-element ultrasoundcatheter tip including the piezoelectric elements of the presentapplication;

FIG. 19 depicts an initial fabrication state of the ink cavity bodyaccording to the concepts of the present application;

FIG. 20 depicts nozzle inputs and an open surface design which may beemployed in the concepts of the present application;

FIG. 21 sets forth an ejector which may be constructed in accordancewith the teachings of the present application; and

FIG. 22 illustrates single ejectors constructed in accordance with thepresent application.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a high level process flow 10 for a first embodimentof a manufacturing process according to the concepts of the presentapplication. While the following discussion focuses on producingpiezoelectric thick film elements, it is to be appreciated the disclosedprocesses may be used with other materials and may also be used forproduction of thin-film elements and elements with thicknesses greaterthan 100 μm to a millimeter scale. Also, the following techniques areintended to be applicable to the generation of individual elements andarrays of elements.

Initially, piezoelectric ceramic thick film, or an array of thick filmelements, is fabricated by depositing the piezoelectric material onto anappropriate substrate by use of a direct marking technology 12. In thedeposition techniques. employed, ceramic type powders are used in apreferred embodiment. The fabrication process includes sintering thematerial preferably at a temperature of approximately 1100 to 1350° C.for desification, although other temperature ranges may also be used inappropriate circumstances. Following the fabrication process the surfaceof the formed structures of piezoelectric elements are polished 14,preferably using a drytape polishing technique. Once the piezoelectricelements have been polished and cleaned, electrodes are deposited on thesurface of the piezoelectric elements 16. Next, the piezoelectricelements are permanently bonded to a final target 18, such as to asubstrate or as part of a larger system. Typically, the composition ofthe piezoelectric ceramic thick film is doped or undoped PZT, but anyother piezoelectric materials, such as lead titanate, lead zirconate,lead magnesium titanate and its solid solutions with lead titanate, leadzinc titanate and its solid solutions with lead titanate, lithiumniobate, lithium tantanate, and others may be used.

At this point, the substrate on which the piezoelectric elements weredeposited is removed through a liftoff process 20 using radiation energysuch as from a laser or other appropriate device. The releasing processinvolves exposure of the piezoelectric elements to a radiation sourcethrough the substrate, to break an attachment interface between thesubstrate and the piezoelectric elements. Additional heating isimplemented, if necessary, to complete removal of the substrate. Oncethe liftoff process has been completed, a second electrode is depositedon a second surface of the piezoelectric material 22. Thereafter, polingof the elements under high voltage obtains piezoelectric properties inthe material 24. The electric property, for example, a dielectricproperty, of each element is then measured 26 to identify if theelements meet required criteria.

Turning to FIG. 2, illustrated is a second high-level process flow 30for a second embodiment of the present application. This process differsfrom FIG. 1 in that the bonding is to a transfer substrate rather thanto a final target substrate or system. Thus, the fabrication step 32,the tape polishing step 34 and the electrode depositing step 36 areperformed in the same manner as steps 12, 14 and 16 of FIG. 1. Atbonding step 38, the bonding is to a transfer substrate, as thisconnection is not intended to be permanent. Thereafter, the liftoff step40, the second electrode deposition step 42, the poling step 44 andelectric property test step 46, which correlate to steps 20, 22, 24 and26 of FIG. 1, are performed.

The piezoelectric elements are then bonded to a final target substrateor system 48, in a procedure similar in design to step 18 of FIG. 1.Following bonding step 48, the transfer substrate is removed 50. Whenbonding to a final target substrate or system, a thin high strengthbonding layer is used to minimize or avoid undesirable mechanicaldamping or absorption of the bonding layer. This bonding will, however,also permit maintaining of electrical contact between the metalelectrodes on the piezoelectric elements and the final target substrateor system or a conductive surface of the final target substrate orsystem.

Employing the process of FIG. 2, only fully tested thick film elementsand arrays will be bonded to final target substrates or systems, thusavoiding yield loss of the target substrates or systems.

The processes of FIGS. 1 and 2 are appropriate for the production ofsingle piezoelectric elements or arrays of the elements, and permit forhigh volume, high usable yields, i.e. greater than 60 percent and morepreferably over 90 percent, and still yet more preferably greater than98 percent.

With attention to FIG. 3, which illustrates steps 12 and 32 in greaterdetail, piezoelectric ceramic elements 52 are deposited on anappropriate substrate 54, and then sintered at 1100 to 1350° C. fordensification. The depositing step may be achieved by a number of directmarking processes including screen printing, jet printing, ballisticaerosol marking (BAM) or acoustic ejection, among others. Using thesetechniques permits flexibility as to the type of piezoelectric elementconfigurations. For example, when the piezoelectric elements are made byscreen printing, the screen printing mask (mesh) can be designed to havevarious shapes or openings resulting in a variety of shapes for thepiezoelectric elements, such as rectangular, square, circular, ring (forannular transducer arrays), among others. Use of these direct markingtechniques also permit generation of very fine patterns.

The substrate used in the processes of this application will havecertain characteristics, due to the high temperatures involved and—aswill be discussed in greater detail—the fact that the substrate is to betransparent for the liftoff process. Specifically, the substrate is tobe transparent at the wavelengths of radiation beam emitted from theradiation source, and is to be inert at the sintering temperatures so asnot to contaminate the piezoelectric materials. A particularlyappropriate substrate is sapphire. Other potential substrate materialsinclude transparent alumina ceramics, aluminum nitride, magnesium oxide,strontium titanate, among others. In one embodiment of the process, thesubstrate selected is transparent for a radiation source, such as anexcimer laser operating at a wavelength of 308 nm, and does not have anyrequirement on its crystallographic orientation. It is preferable thatthe selected substrate material be reusable, which will provide aneconomic benefit to the process.

After fabrication of the elements has been completed, the process movesto step 14 (or 34), where the top surface of the piezoelectric elementsare polished through a tape polishing process to remove any surfacedamage layer, such as due to lead deficiency. This step ensures thequality of the piezoelectric elements and homogenizes the thickness ofpiezoelectric elements. By having a homogenized thickness, each of thepiezoelectric elements of an array will bond to the final target systemor the transfer substrate even when a very thin epoxy bonding layer or athin film intermetallic transient liquid phase bonding layer is used.

In one preferred embodiment, the tape polishing step is a dry tapepolishing process that provides a planar flat polish out to the edge ofthe surfaces of the piezoelectric elements, which avoids a crowningeffect on the individual elements. Compared to a wet polishingprocesses, the dry tape polishing does not cause wearing of the edges ofthe piezoelectric elements, making it possible to fabricatehigh-quality, thickness and shape-identical piezoelectric elements. Oncepolishing has been completed, the surface is cleaned, in one instance byapplication of a cleaning substance.

After polishing and cleaning, the process moves to step 16 (or 36)where, as shown in FIG. 4A, metal electrodes 56 such as Cr/Ni or otherappropriate materials, are deposited on the surface of the piezoelectricelements by techniques such as sputtering or evaporation with a shadowmask. The electrode can also be deposited a direct marking method, suchas screen printing, and sintered at suitable temperatures.

Alternatively, when using a thin film intermetallic transient liquidphase bonding process, certain low/high melting-point metal thin filmlayers maybe used as the electrodes for the piezoelectric elements, thusin some cases it is not necessary to deposit the extra electrode layersuch as Cr/Ni. However, preferably the thin film intermetallic transientliquid phase bonding process is undertaken after metal electrodedeposition, such as Cr/Ni deposition. While this process will bediscussed in greater detail below, generally a thin film layer of highmelting-point metal 58 (such as silver (Ag), gold (Au), Copper (Cu),Palladium (Pd)) and a thin film layer of low melting-point metal 59(such as Indium (In), Tin (Sn)) may be deposited on the piezoelectricelements (or the substrate) and a thin layer of high melting-point metal(such as Ag, Au, Cu, Pd) may be deposited on the substrate (or thepiezoelectric elements). These materials are then used to form a bond.Also a multilayer structure with alternating low melting-pointmetal/high melting-point metal thin film layers can be used.

For some uses, such as when the final target substrate or system is notexpensive, the piezoelectric elements are directly bonded to the finaltarget substrate or system (step 18 of FIG. 1). For example, as depictedin FIGS. 5A and 5B, the final target could be a metal foil (also used ascommon electrode) 62, which is put on a carrier plate 60 during theprocess. The bonding is accomplished by using a nonconductive epoxylayer 64 which can be as thin as less than 1 μm. The thin epoxy containssub-μm conductive particles, which in one embodiment may be conductiveballs (such as Au balls) 65 so the epoxy is conductive in the Zdirection (the direction perpendicular to the surface of metal foil).Thus it can keep the electric contact between the surface electrode ofthe piezoelectric elements and the metal foil. The concentration of theconductive balls can be controlled in such a range that the cured thinepoxy is conductive in the Z direction but not conductive in the lateraldirections, as done for the anisotropic conductive films. The shrinkageof the epoxy maintains contact between the surfaces and the balls in theZ direction.

In an alternative embodiment shown in FIGS. 5B and 5C, conductive balls65 are removed, and bonding is accomplished using the nonconductiveepoxy layer 64 alone. As shown in more detail by FIG. 5C, electricalcontact is maintained via electrical contact points 66, formed when thesurface of the electrode 56 and metal foil 62 are moved into contact,with suitable surface roughness or asperity of the piezoelectric filmsand/or metal foil.

In a further embodiment, bonding to the final target maybe accomplishedby using the previously mentioned thin film intermetallic transientliquid phase bonding, employing in one embodiment a high melting-pointmetal (such as Ag, Cu, Pd, Au, etc.)/low melting-point metal (such asIn, Sn) intermetallic compound bonding layer or alloy 68, FIG. 5D.

More particularly, for thin film intermetallic transient liquid phasemetal bonding, a high melting-point metal thin layer, such as a Pd thinlayer, is deposited on the target substrate or system. Next thepiezoelectric elements are moved into contact with the Pd thin layer andheated under pressure above the melting point of the low melting-pointmetal, e.g., about 200° C. By this operation the high melting-pointmetal/low melting-point metal/high melting-point metal combination, suchas Pd/In/Pd layer (a high melting-point metal/low melting-point metalsuch as Pd/In layer was previously deposited on the piezoelectricelements as shown in FIG. 4B) will form the high melting-point metal-lowmelting-point metal bonding layer compound or alloy 68. This compound oralloy may be a PdIn₃ alloy layer which is about 1 μm-thick, which actsto bond piezoelectric elements 52 and target substrate or system 62.Functionally, the low melting-point metal diffuses into the highmelting-point metal to form the compound/alloy.

As the melting point of the formed intermetallic compound phase can bemuch higher than that of the low melting-point metal, the workingtemperature of the bonding layer can be much higher than the temperatureused to form the bonding. For example, when Indium (In) is used as thelow melting-point metal and Palladium (Pd) is used as the highmelting-point metal, the bonding can be finished below or at 200° C. asthe melting point of In is about 156° C. However, the workingtemperature of the formed intermetallic compound bonding layer, PdIn₃,can be well above 200° C. because the melting point of PdIn₃ is about664° C. The thickness of the bonding layer could be from 1 to 10 μm, buta thinner bonding layer (e.g., about 1 μm) is expected for this purpose.Further, the amount of high and low melting-point metals can becontrolled so they will be totally consumed to form the intermetallicbonding layer.

In some situations, a final system may be larger than a substrate onwhich elements are deposited. In other situations, different types ofpiezoelectric materials—such as soft PZT and hard PZT—or piezoelectricmaterial and other ceramic material, are intended to be transferred tothe same final target system. In either of these instances, as shown inFIG. 5E, the materials can be deposited on separate substrates, or eachdifferent piezoelectric material, or each piezoelectric material andeach other ceramic material can be deposited on each substrateseparately, then sintered at suitable temperatures. Then each of theelements may have electrodes deposited on their surfaces. After that,substrates with different material can be bonded to the same finalsystem. For example, in FIG. 5E two substrates—a first substrate 54deposited with piezoelectric elements 52, and a second substrate 55deposited with elements 53 (which could be piezoelectric or otherceramic materials such as antiferroelectric material)—are bonded to thesame final substrate 62 using thin epoxy bonding 64 containing sub-μmconductive balls 65. Clearly, similar cases can also be applied to useother bonding methods, and to the case where a transfer substrate isused rather than directly bonded to the final target system, such as isdescribed in detail later. When the process with transfer substrate isused, several transfer substrates can also be used.

Alternatively, when the final target substrate or system is expensive,bonding of the piezoelectric elements to the final target is delayed.Incorporation of the steps in FIG. 2 minimizes yield loss of the finaltarget substrate or system, which might otherwise occur due topiezoelectric film fabrication failures. Examples of where this processmay be implemented include the manufacture of micro-fluid pumps, such asjet printheads and acoustic ejectors, or silicon wafers havingcomplicated pattern configurations and electric circuits. Thenon-piezoelectric components of these devices can be ten times moreexpensive than the piezoelectric materials. Therefore, the process ofFIG. 2 temporarily bonds the piezoelectric elements to a transfersubstrate in step 38, and then finishes piezoelectric film productionand testing. Only a fully tested piezoelectric thick film element orarray of elements is then permanently bonded to the target substrate orsystem.

The temporary bonding process step 38 of FIG. 2, is illustrated by FIGS.6A and 6B. In FIG. 6A, the bonding operation uses a removable conductivebonding epoxy, such as a removable conductive tape 70, including9712,9713 and 9719 conductive tape from 3M Corporation. The transfersubstrate 72 can be a metalized glass with surface conductive layer 74,such as a metalization layer. In an alternative embodiment depicted inFIG. 6B, the bonding operation uses thin nonconductive epoxy 64containing sub-μm conductive balls 65, to bond to a transfer substrate78 such as a glass having an ITO coating 80.

It is noted that to manufacture ready-to-use single piezoelectric thickfilm elements as the final product, the individual piezoelectricelements will also be bonded to a transfer substrate.

Once the piezoelectric elements have been either permanently bonded to afinal target substrate or system (step 18 of FIG. 1) or temporarilybonded to a transfer substrate (step 38 of FIG. 2), the next step is torelease the piezoelectric elements 52 from substrate 54. The releasingof substrate 54 is accomplished by a liftoff operation as depicted inFIGS. 7A and 7B. The following description is based on the arrangementof FIG. 5A. However, it is applicable to all provided alternatives.Substrate 54 is first exposed to a radiation beam (such as a laser beam)from a radiation source (such as an excimer laser source) 82, having awavelength at which the substrate 54 is substantially transparent. Inthis way a high percentage of the radiation beam passes through thesubstrate 54 to the interface of the substrate and elements 52 at thesurface of the substrate. The energy at the interface acts to break downthe physical attachment between these components. Following operation ofthe radiation exposure, and as shown in FIG. 7B, heat is applied by aheater 84. While the temperature provided by the heater will varydepending on the situation, in one embodiment a temperature of between40 to 50° C. is sufficient to provide easy detachment of any remainingcontacts to fully release the piezoelectric elements 52 from substrate54. Desirably, the substrate is of a material that allows it to bere-used after a cleaning of its surface.

In one experiment performed by the inventors, the radiation source is anexcimer laser source and the laser energy required to achieve separationby the present procedure has been measured at about one-half what ismentioned as needed in the Cheung et al. patent. This is considered inpart due to the wavelength used in the experiment (e.g., 308 nm), andalso that the piezoelectric material is polycrystalline and was screenprinted on substrates, therefore more weakly bound to the substratecompared to the epitaxially grown single crystal films used in theprevious work by Cheung et al.

Exposure to the radiation source does raise the potential of damage tothe surface of the piezoelectric elements, this potential damage shouldhowever be no more than to a thickness of about 0.1 μm. Since thethickness of the piezoelectric elements, in most embodiments, will belarger than 10 μm, the effect of the surface damage layer can beignored. However, if otherwise necessary or when piezoelectric elementsof less than 10 μm are formed by these processes, any surface damagelayer can be removed by appropriate processes including ion milling ortape polishing. It is to be appreciated FIGS. 7A and 7B are simply usedas examples, and the described liftoff process may take place usingalternatively described arrangements. Also, for convenience, FIGS. 7Aand 7B corresponds to the structure of FIG. 5A. However, the same typesof procedures may be applied to FIGS. 5B to 5E, FIGS. 6A and 6B or otherrelevant arrangements in accord with the present teachings.

Next, as depicted in FIGS. 8A and 8B, second side surface electrodes 86,such as Cr/Ni, are deposited on the released surfaces of elements 52with a shadow mask or by other appropriate method in accordance withstep 22 of FIG. 1 or step 42 of FIG. 2. After second electrodedeposition, the processes move to steps 24 and 44, respectively, wherethe piezoelectric elements 52 are poled under a voltage 88 sufficient,as known in the art, to obtain piezoelectric properties. After poling,the electric property, for example, the dielectric property, of theelements are measured (step 26 of FIG. 1; step 46 of FIG. 2) to identifyif the piezoelectric elements meet expected quality criteria. FIG. 8Acorresponds to the arrangement shown in FIG. 5A, and FIG. 8B correspondsto the arrangement of FIG. 6A.

For the case where a piezoelectric thick film element or array ofelements is already bonded to the final target substrate or system suchas by the process of FIG. 1, this is the final step of the process. Forthe case where the piezoelectric thick film element or array of elementsis temporally bonded to a transfer substrate such as by the process ofFIG. 2, steps 48 and 50 are undertaken. In the following these steps areimplemented using selected ones of the alternative arrangementspreviously described. It is to be understood the discussion inconnection with these alternatives are applicable for all disclosedalternative designs.

As mentioned, the piezoelectric element or array of elements istemporally bonded to a transfer substrate in situations where, forexample, the final target substrate or system is much more expensivethan the piezoelectric thick film elements. By use of this temporarybonding, it is only after electric property measurement is made that thepiezoelectric element or array is bonded to the final target.

Step 48 of FIG. 2 may be accomplished in the same manner as bonding step18 of FIG. 1. FIGS. 9A-9D, show alternative bonding methods, including athin nonconductive epoxy bonding containing sub-μm conductive balls(FIG. 5A) and a thin film intermetallic transient liquid phase bonding(FIG. 5D). Still further, the process could employ the thinnonconductive epoxy bonding of FIGS. 5B and 5C. When this process isused, the surface roughness or asperity of the piezoelectric elements/orand the substrate is preferably in a range of about 0.5 to 5 μm,depending on film thickness, nature of the substrate, as well as theintended use. The second surface of the piezoelectric elements could bevery smooth due to the smooth nature of the substrate surface. Thismeans that, after liftoff, rough tape polishing, sandblasting or othermethods may be needed to increase the surface roughness. It is to beunderstood the surface roughness will be a small fraction of the overallthickness of the piezoelectric element and/or substrate. The specificroughness being selected in accordance with a particular implementation.

If the thin film intermetallic transient liquid phase bonding is used,similar to previous steps, a high melting-point metal/low melting-pointmetal such as Pd/In electrode is deposited on the second surface of thethick film elements and a thin high melting-point metal such as Pd layeris deposited on the surface of the final target system.

It is to be appreciated the surface of the final target system is to beconductive. Therefore, either the body of the final target system isconductive, such as a stainless steel printhead, or the surface of thefinal target system is conductive, such as metalized silicon wafers forMEMS applications. Further, FIGS. 9A-9D are related to the process ofFIG. 2, where the first bonding step is to a temporary connection.

With more particular attention to FIG. 9A, to bond the thick filmpiezoelectric elements 52 to final target 90, nonconductive epoxy 64containing sub-μm conductive balls 65 is interposed between a surface ofthe conductive layer 96 of the final target 90 and thick film elements52. The opposite side surfaces of the thick film elements 52 are alreadytemporarily bonded to the transfer substrate 72 (via conductor 74)through the use of a removable conductive tape 70.

FIG. 9B illustrates an alternative bonding of the thick film elements 52to final target system 90 using thin film intermetallic transient liquidphase bonding 68, where the thick film elements 52 are bonded to thetransfer substrate 72 using removable conductive tape 70.

The alternative bonding of FIG. 9C, shows the thick film elements 52bonded to the final target system 90 using thin nonconductive epoxybonding 64 containing sub-μm conductive balls 65. In this design,elements 52 are bonded to an ITO coated 80 glass substrate 78 using thethin nonconductive epoxy 64 containing sub-μm conductive balls 65.

Depicted in FIG. 9D is an arrangement where the elements 52 are bondedto the final target system 90 (via conductor 96) using thin filmintermetallic transient liquid phase bonding 68, where the thick filmelements 52 are bonded to ITO coated 80 glass 78 using the thinnonconductive epoxy 64 containing sub-μm conductive balls 65.

Once the final target has been bonded to the elements, the processproceeds to step 50 and the transfer substrates (such as 72 or 78) areremoved, as shown in FIGS. 10A and 10B. For the case where the thickfilm elements are bonded to the transfer substrate using removableconductive epoxy, such as tape, after permanent bonding to the finaltarget system is achieved, the tape and the transfer substrate can beeasily peeled off from the thick film elements. The present processmakes it easy to take off the conductive tape. This is because theconductive tape uses filled acrylic, such as the 3M 9712, 9713 and 9719conductive tapes, which lose most of their adhesion after being heatedat a temperature of between 150 and 200° C. The time needed forapplication of the heat will depend upon the specific application. Insome applications this level of heat maybe applied during the process tobond the thick film elements 52 to the final target system or substrate.

For the case where the thick film elements 52 are bonded to the ITOcoated glass using the thin nonconductive epoxy, the film elements canbe released from the ITO coated glass by using a liftoff operation in amanner similar as in steps 20 or 40, where the radiation source is alaser. This is possible as the epoxy will also absorb the laser light,thus the laser exposure will burn off the epoxy and release the filmfrom the glass substrate. As the melting point of epoxy is much lowerthan that of the metal and ITO electrodes, the laser exposure intensitymaybe controlled so it will only burn off the epoxy and not cause anydamage on the metal and ITO electrodes.

After removing the transfer carrier, solvent such as acetone or otherappropriate substance may be used to clean off the residual of theconductive tape or the epoxy, and the process is completed.

It should be noted that when using laser liftoff techniques to releasethe piezoelectric thick film elements from ITO-coated glass, in oneembodiment an excimer laser with relatively longer wavelength, such asNd:YAG laser (λ=355 nm) and XeF (λ=351 nm) is to be used. This isbecause, as shown in FIG. 11 , the transmission of light through ITO onglass will drop sharply around λ=300 nm, but around λ=350 nm thetransmission can be about 80%. With such high transmission, the laserexposure can be controlled so that only the epoxy is destroyed anddamage to the ITO and metal electrodes does not occur.

When the final target is a single piezoelectric ceramic thick filmelement—such as for single element high frequency transducers—it isdesirable to put the single thick film elements on a rigid carrier usingremovable tape, which does not need to be conductive. In this situation,therefore, step 48 of FIG. 2 may be altered.

Particularly, where as in FIG. 12, removable conductive tape 70 has beenused to bond the thick film elements 52 to the transfer substrate 70,the piezoelectric elements 52, and the transfer substrate 72 are heatedto about 150 to 200° C. or other appropriate temperature causing theremovable conductive tape 70 to lose most of its adhesion. Adhesion ofthe tape may be further reduced by putting the sample in a solvent suchas acetone. Then the thick film elements are stuck to a rigid carrier112 using removable tape 114. For the case where, as in FIG. 13, thinnonconductive epoxy 64 is used to bond the thick film elements 52 to anITO 80 coated glass 78 as the transfer substrate, and the thick filmelements are stuck to the rigid carrier 112 using removable tape 114. Inthis case, the transfer substrate maybe removed by the previouslydiscussed liftoff process.

After taking off the transfer substrate, solvents such as acetone may beused to clean off the residual of the conductive tape or the epoxy. Nowthe piezoelectric ceramic thick film elements are on a rigid carrier andare ready for use as shown in FIG. 14.

The proposed processes can be applied to make piezoelectric thick filmarrays or individual piezoelectric elements for a variety of uses suchas microfluid pumps including jet printers or acoustic ejectors, as wellas for MEMS, high frequency transducers, catheters and other structures.Particular ones of these structures are now discussed.

With attention to microfluid pumps, it is known that current printheadscommonly use bulk piezoelectric ceramics and make the actuator arrays bysaw cutting. There are several drawbacks to this process: i) theperformance of the actuation system (piezoelectric element+stainlesssteel diaphragm) cannot be optimized. With the thickness of thestainless steel diaphragm of 25 to 40 μm, the thickness of thepiezoelectric elements should be about 40 to 70 μm for an optimizeddesign. However, the thickness of the bulk piezoelectric elements is 100μm or thicker as ceramic industry cannot now easily make bulkpiezoelectric ceramics thinner than 100 μm; ii) only rectangular orsquare shapes can be realized by saw cutting, and this greatly limitsthe design feasibility; iii) cost is high, due to the time-consumingprocess and equipment cost of the saw cutting process, and very highrequirements on bulk ceramics so that they can be cut into thin andsmall pieces. For example, for a printhead which needs 155×8piezoelectric elements for one printhead, this means the manufacturingprocess needs to do 154+7=161 times of cutting for just one printhead;and iv) due to the saw cutting process, it is not possible to make highnozzle densities. It can be seen that using the method proposed in thisapplication, the problems related to use of bulk piezoelectric ceramicsand saw cutting process can be solved.

To use the proposed method to make piezoelectric thick film elementsarray for a microfluid pump, such as a printhead, either the transfersubstrate process may be used or the thick film array may be permanentlybonded to a metal foil such as copper foil. When the transfer substrateprocess is used, we will first transfer the piezoelectric thick filmelements array from the carrier substrate to the transfer substrate,finish the piezoelectric film fabrication and property test, then bondthe thick film elements to the printhead and release them from thetransfer substrate. For this approach the current return path is thestainless steel diaphragm, which exists in current printheadconfigurations.

A printhead 120 as shown in FIG. 15 is formed on the stainless steeldiaphragm 124, and the stainless steel is covered by an insulating layer126, thus the top surface of piezoelectric thick film element (oractuators) 128 have to be connected as a current return path. Toaccomplish this, the piezoelectric thick film elements 128 arepermanently bonded on a metal foil 130, such as copper, then releasedfrom the substrate (not shown) and the piezoelectric fabrication isfinished. After testing, the piezoelectric elements are bonded to theprinthead body 132, and thus the metal foil 130 will be on top of thepiezoelectric thick film elements 128 and can be used as the currentreturn path.

Another use to which the piezoelectric elements may be applied to isintegrated hand-held sonar transducer arrays.

FIG. 16 is a side view of a micromachined transducer 134 which can beused as hand-held sonar, for ultrasound medical imaging andnondestructive testing, etc. Existing devices of this kind use 5μm-thick sol-gel piezoelectric PZT films on a patterned silicon-baseddiaphragm to make bending mode transducers. However, to operate athigher frequencies, increase the voltage sensitivity or make the devicework as both a transmitter and receiver, piezoelectric films with athickness more than 10 μm are required, such as the piezoelectric thickfilm elements array disclosed in the present application.

The piezoelectric element 136 is sandwiched on top by a polyimide layer138 which in turn carries a top Ti/Pt layer 140 and provides for amonomorph contact 142 and substrate contact 144. Sandwiching thepiezoelectric element 136 on a bottom surface side is a bottom Ti/Ptlayer 146, which separates the piezoelectric layer 136 from a SiO₂ layer148. Transducer 134 is further configured with a bottom SiO₂ layer 150,and layers of P+ silicon 152, 154 are formed on each side of an n-typesilicon 156. An etched cavity 158 completes the design configuration.

A further application to which the concepts of the present applicationmay be used are arrays and single elements for high frequencytransducers 160 (FIG. 17) and catheters 170 (FIG. 18). High frequency(e.g., 20 to 200 MHz) transducers and catheters are widely used inimaging skin, arterial walls, structures in the anterior chamber of theeye, and intravascular ultrasound imaging. To make these transducers,piezoelectric materials with thickness between greater than 10 and 100μm are desirable, due to the resonant frequencies which may be obtainedin this range. Table 1 gives the resonant frequency of piezoelectricfilms with various thickness (for longitudinal mode) in a range fromgreater than 10 to 100 μm. TABLE 1 Resonant Frequency of piezoelectricMaterials With Various Thicknesses Piezoelectric thickness (μm) 100 5040 20 10 Resonant Frequency 20 40 50 100 200 (MHz)

As previously noted, it is difficult to make piezoelectric materialsthinner than 100 μm, where the current method used in industry is topolish down the thickness of piezoelectric materials from more than 100μm to the required thickness. This makes the piezoelectric elementexpensive. The polishing down bulk piezoelectric materials is alsodifficult to make some high frequency transducer arrays, such as theannular arrays 162 of (annular transducer 160) FIG. 17.

Clearly the method proposed in this application can easily make thepiezoelectric thick film single elements and arrays for theseapplications, including the complicated arrays such as the annulararray.

Mechanical rotating single-element catheters 170 may be constructed witha rotating shaft 172, a transparent dome 174 and a transducer element176, among other known components. The transducer element ispiezoelectric material preferably with a thickness of between 10-100 μm,and preferably 50 μm in order to obtain a working frequency of 40 MHz.

A further structure to which a piezoelectric elements array as describedin the present application may be applied is to a microfluid ejector orpump, such as a jet printhead, acoustic ejector or other drop ejectionmechanism as shown in FIGS. 19-21.

Existing commercialized piezoelectric ejectors will commonly use bulkpiezoelectric ceramics as actuators and stainless steel for the inkcavity body and nozzle or control level plate. In one design, the inkcavity body and control or nozzle plate is made from many pieces orlayers of stainless steel sheets which are blazed together under hightemperature. An alternative type of ejector or printhead is developedusing piezoelectric thin film actuators and silicon as the ink cavitybody, made by silicon micromachining. The piezoelectric thin films canbe made by sol-gel, sputtering, hydrothermal processing, among others.

However, drawbacks to these types of devices are their expense, or theirinability to generate sufficient force to eject a droplet of a requiredsize. For instance, in biofluid printing, it may be desired that theejector system is disposed after each use, which would therefore callfor an inexpensive drop ejection mechanism. Additionally, as thepiezoelectric films in existing systems are very thin (less than 10 μm),the actuator in some applications such as solid-state ink printing andbioprinting, may not provide sufficient energy for proper dropletejection. Up until now, there has not been a cost-effective method tocombine piezoelectric thick films on stainless steel or siliconsubstrates.

A specific aspect of the present embodiment is to make piezoelectricthick films (i.e., between greater than 10 to 100 μm) as actuators, andto combine these thick film elements to a plastic ink cavity body andnozzle (e.g., for jet printheads) or liquid control plate (e.g., foracoustic ejection). Such a design provides an economic advantage overexisting systems employing stainless steel stacks or silicon andsimplifies the manufacturing process via the use of the plasticmaterial.

Initially, the process of FIG. 2 is undertaken, whereby piezoelectricthick film actuators are manufactured. A generic view might be seen asFIGS. 10A and 10B, where in this embodiment the final system will be aplastic ink body cavity and nozzle or control plate design.

More particularly, as shown in FIG. 19, a plastic workpiece 180 isformed by injection molding, or other appropriate plastic manufacturingprocess, with ink cavities 182 in the body. As shown, the ink cavity inthis embodiment has a dome or other curved shape so that the bottom partof ink cavity 182 has a thinner thickness than found at the side walls.This design allows the manufacture of a nozzle in the thinnest part ofthe ink cavity body, where the ink cavity wall becomes thicker when itis away from the nozzle area. This design is desirable when using theplastic design, as the plastic is softer than steel and silicon andavoids the forming of a thin nozzle plate.

Following the injection molding process, workpiece 180 is then furthermanufactured by forming nozzle holes 184 at the thinnest part of theplastic ink cavity body 182, as shown in FIG. 20. In one form of theprocess, the nozzle holes 184 are made by the use of laser cutting.However, other processes to make nozzle holes 184 may be employed. It isto be appreciated, that in alternative embodiments (e.g., when used foracoustic ejection), the workpiece 180 will not require nozzle holes 184.Rather, in some embodiments liquid control plates may be used. Thus, thesystem when used for acoustic ejection may have an open surface orliquid control plate. In FIG. 20 dotted lines 186 illustrate theformation of the workpiece 180 when used with acoustic ejection.

Following the formation of the workpiece 180, a next step includesbonding the plastic workpiece to a metal diaphragm with actuators, whichmay be accomplished via the steps described in connection with FIG. 2.

Illustrated in FIG. 21 is a basic ejection or printhead structure 188,configured according to the concepts of the present application. In thisdesign, the piezoelectric thick film elements 190, which are preferablybetween greater than 10 to 100 μm are the actuators. In alternativeembodiments, the actuators maybe 10 μm or less or greater than 100 μm.The actuators attached to metal diaphragm 192 are then connected, viathe metal diaphragm, to the workpiece 180. Bonding layers 194, 196 maybe formed by any of the previously described appropriate bondingtechniques. Bonding layer 196 can also be a thick epoxy bonding process,as maintaining electric conductivity between the workpiece 180 and metaldiaphragm 192 is not required. Electrical contacts 198 permit supplyingof power to the printhead 188.

Plastic workpiece 180 is designed with an ink cavity body having anozzle or liquid control plate arrangement. By this design, when in usewith a printhead, plastic is used to make the ink cavity body and nozzledirectly on the actuator arrangement, not requiring a separate nozzle orplate. The lateral dimension of the piezoelectric thick film actuatorsmay be either the same or different between each other. The channel sizemay also be either the same or different between the other channels.

As previously noted, and with reference to FIG. 21, the plastic cavitybody formed with the nozzle, may be made by injection molding and/orlaser cutting. Also provided is a thin metal layer such as a stainlesssteel sheet which works as a passive diaphragm, and the piezoelectricfilms which work as the actuators. The thickness of the steel sheet maybe from about 20 μm to 100 μm, and the piezoelectric film thicknesscould be from about greater than 10 to 100 μm. The shape of thepiezoelectric film elements may be square, rectangular, circular orother forms.

In addition to the arrays of ejectors as shown in FIG. 21, it is to beappreciated that a single ejector 200 as shown in FIG. 22, may bedesigned as the final product. Each ejector 200 will be made in asimilar manner as described, but with sufficient area so a cut line 202may be used to obtain single ejector units.

It is to be appreciated, while one embodiment of the piezoelectricelement array or single elements are shown in FIGS. 21 and 22,alternative elements or components may be used to generate the disclosedstructures.

By the proposed disclosed processes, a fast efficient manner of makinghigh volume, piezoelectric ceramic thick film arrays or single elementsin a thickness range from greater than 10 to 100 μm is disclosed. It isto be appreciated that by use of an appropriate marking technique, suchas screen printing, the range may be extended to as low as 5 μm orlower, and above 100 μm.

In this process, it is also known to be possible the elements may bepatterned in any arbitrary geometric shape. Also, as only solid statepowders are used as the raw materials, and the substrate is reusable,such as a sapphire substrate, economic advantages to the process overexisting techniques are achieved.

It is also noted that by use of the proposed processes, a clean and lowtemperature technique for attachment to final target substrates orsystems, irrespective of the material for the final target or substrateor system is achieved. For example, this process is fully compatiblewith IC processes, if the final system is to be a silicon basedmicroelectronic device. It has been experimentally demonstrated by theinventors that bonding the piezoelectric films to a silicon wafer andperforming the liftoff procedures does not damage a CMOS circuit on asilicon wafer.

Additionally, by the process of FIG. 2, the piezoelectric filmfabrication process, including the poling step that causes the mostdamage in yield loss of piezoelectric films, are performed when thepiezoelectric film is separated from the final target substrate orsystem. This further permits the piezoelectric thick film elements to betested before being bonded to the final target substrate or system.Therefore, the proposed process will result in very low yield losses forthe final target substrate or system.

The invention has been described with reference to the preferredembodiments. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

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 33. (canceled)34. A method of producing at least one thick-film element comprising:depositing a material onto a surface of at least one first substrate toform at least one thick-film element structure having a thickness ofbetween approximately greater than 10 μm to 100 μm; bonding the at leastone thick-film element structure to a second substrate; removing the atleast one first substrate from the at least one thick-film elementstructure using a liftoff process using radiation energy including,emitting, from a radiation source, a radiation beam through the firstsubstrate to an attachment interface formed between the first substrateand the at least one thick-film element structure at the surface of thefirst substrate, wherein the first substrate is substantiallytransparent at the wavelength of the radiation beam, permitting theradiation beam to generate sufficient energy at the interface to breakthe attachment.
 35. The method according to claim 34, wherein theradiation beam is a laser beam.
 36. The method according to claim 34,wherein the step of removing the first substrate further includesheating the interface with a heater to break any remaining attachment.37. The method according to claim 34, wherein the first substrate isreusable.
 38. The method according to claim 34, wherein a step ofdepositing an electrode on a surface of the at least one thick-filmelement structure, is undertaken following the step of depositing amaterial onto a surface of the first substrate to form the at least onethick-film element structure.
 39. The method according to claim 34,wherein the bonding step includes a step of providing a nonconductiveepoxy bonding containing conductive particles between the electrode andthe second substrate, or providing a nonconductive epoxy bonding alone,the second substrate being an electrically conductive substrate orhaving an electrically conductive portion, wherein the nonconductiveepoxy bonding process provides for electrical contact between theelectrode and the second substrate.
 40. The method according to claim34, wherein if more than one first substrate is used, the material onthese substrates could be either the same or different.
 41. The methodaccording to claim 34, wherein if more than one substrate is used, thethickness and geometrical shape of the elements on the substrates arethe same.
 42. The method according to claim 34, wherein if more than onesubstrate is used, the thickness and geometrical shape of the elementson the substrates are different.
 43. The method according to claim 34,wherein the depositing step includes patterning the at least one thickfilm elements.
 44. The method according to claim 34, wherein thedepositing step implements a direct marking process.
 45. The methodaccording to claim 44, wherein the direct marking process is at leastone of screen printing, ballistic aerosol marking, jet printing andacoustic ejection.
 46. The method according to claim 34, wherein thedepositing step includes sintering the at least one thick film elementstructure at a temperature of above 600° C.
 47. The method according toclaim 34, wherein the depositing step includes sintering the at leastone thick film element structure at a temperature in a range ofapproximately 1100° C. to 1350° C.
 48. The method according to claim 34,further including polishing a surface of the at least one thick filmelement structure.
 49. The method according to claim 48, wherein thepolishing step is a dry tape polishing procedure.
 50. The methodaccording to claim 34, wherein the wavelength of the radiation beam isapproximately 308 nm.
 51. The method of claim 34, wherein the materialdeposited onto the first substrate is a polycrystalline and it wasdeposited by a screen printing process.
 52. The method of claim 34,wherein exposure to the radiation source results in damage to thesurface of the element, the damage being no more than to a thickness ofabout 0.1 μm.
 53. The method of claim 36, wherein the heater generates atemperature of between 40° C. to 50° C.